Many electronic devices are configured to determine position using satellite positioning system(s). To determine the position, the electronic device acquires signals from a plurality of satellites orbiting the Earth. Using information contained in these signals, the device or an adjunct device, such as a position determining entity, calculates the position or location of the electronic device.
As can be appreciated, the devices must acquire satellite signals in order to use the signal. Acquisition of satellite signals can be difficult as the signal strength is frequently attenuated and weak at the Earth's surface. For example, the Global Positioning System, which is one example of a Satellite Positioning System, provides a signal strength only slightly above the minimum signal acquisition strengths of most devices.
“Acquisition” refers to differentiation of the signal from a particular source (e.g., a first satellite in a first satellite system) from other signals. In the GPS satellite system, this is accomplished by determining the relative phase of the coarse acquisition (C/A) pseudorandom noise (PN) code for a particular satellite at the receiver. In general, the receiver searches in code phase space by generating copies of the satellite's PN code at different relative offsets and correlating the received signal with the generated code. Peaks in the power of the correlation result generally designate the code phase offset (although effects like multipath can make analysis more difficult). Additionally, because the satellite is moving with respect to the receiver or the receiver is moving with respect to the satellite, a search in frequency space can be performed to account for the Doppler Effect. Doppler changes the frequency of the carrier at the receiver, as well as effectively increasing or decreasing the duration of each chip of the PN code.
To facilitate acquisition of the satellite signals, some conventional satellite receivers include a sequential detection algorithm to acquire the code division multiple access (generally known as CDMA) signal (e.g., the PN code for a GPS satellite). The detection algorithm allows lower acquisition thresholds by measuring the power of a correlation signal over a predetermined length of time. If the power of the signal is over a predefined threshold, it is considered a satellite signal instead of noise. Noise signals, as can be appreciated, would tend to have 0 or close to 0 power when correlated with a generated code signal over a length of time. Measuring power over a period of time is generally known in the art and includes both a coherent integration time (sometimes referred to as “CIT”) and a non-coherent integration period (a number of times that coherent integration will be performed). The coherent integration time is also commonly known as the pre-detection interval, while the non-coherent integration is commonly known as post detection interval.
Conventionally, coherent integration times are set at approximately 1 to 20 milliseconds for GPS when the data bits are not known. For GPS, the C/A code, a government precision code (P code), and a navigation message are transmitted on the L1 carrier frequency (1575.42 MHz). The C/A code is 1023 chips long and approximately a millisecond in length, so that each code chip has a duration of about a microsecond. The navigation message has a relatively low data rate; at 50 bits/second, the duration of each bit is about 20 milliseconds. Therefore, correlation of the received signal with the generated PN code may be unpredictably interrupted by a signal reversal at the next bit boundary (if the message has a transition from one to the other of the binary states). It may be advantageous to provide even longer coherent integration times to maximize signal acquisition. Longer coherent integration times provide numerous advantages and numerous disadvantages. Advantages of longer coherent integration times include, for example, reduction of the in-band noise and provides more integration (i.e., longer integration time increases the signal power of the correlation result over the interval, allowing better discrimination of the signal from the satellite). Disadvantages of longer coherent integration times include, for example, a potential of signal reversal due to transmitted data bits (which causes signal power over the interval to decease), reduction in frequency coverage, and potential loss of satellite signal information.
For example, conventional technology may set the coherent integration time at 20 milliseconds. If, for example, the number of non-coherent integration intervals is set at 50, the total integration time of the signal provides a certain amount of energy, which may or may not be sufficient to acquire a weak signal. Increasing the number of non-coherent integrations to, for example, 100, increases the gain by approximately 1.5 dB, but significantly lengthens (doubles) the total integration time. Extending the coherent integration time to 40 milliseconds and decreasing the number of non-coherent integrations to 25 also increases the total gain by approximately 1.5 dB, without significantly lengthening the total integration time. While less integration time and greater energy capture provide significant advantages, consistently providing a coherent integration time longer than 20 milliseconds risks the potential for signal loss.
Against this background, it would be desirous to provide an adaptive coherent integration time that could be shortened and lengthen depending on the circumstances.